arXiv subjects

R. W. Schmidt

Publications and source records attributed to R. W. Schmidt.

At least 37 records · Page 2Linked to original sources

arXiv2013

Strong lensing in RX J1347.5-1145 revisited

We present a revised strong lensing mass reconstruction of the galaxy cluster RX J1347.5-1145. The X-ray luminous cluster at redshift z=0.451 has already been studied intensively in the past. Based on information of two such previous (strong-)lensing studies by Halkola et al. (2008) and Bradac et al. (2008), as well as by incorporating newly available data from the Cluster Lensing And Supernovae survey with Hubble (CLASH, Postman et al. 2012), we identified four systems of multiply lensed images (anew) in the redshift range 1.75 <= z <= 4.19. One multiple image system consists of in total eight multiply lensed images of the same source. The analysis based on a parametric mass model derived with the software glafic (Oguri 2010) suggests that the high image multiplicity is due to the source (z_phot = 4.19) being located on a so-called "swallowtail" caustic. In addition to the parametric mass model, we also employed a non-parametric approach using the software PixeLens (Saha and Williams 1997, 2004) in order to reconstruct the projected mass of the cluster using the same strong lensing data input. Both reconstructed mass models agree in revealing several mass components and a highly elliptic shape of the mass distribution. Furthermore, the projected mass inside, for example, a radius R ~35 arcsec ~200 kpc of the cluster for a source at redshift z=1.75 obtained with PixeLens exceeds the glafic estimate within the same radius by about 13 per cent. The difference could be related to the fundamental degeneracy involved when constraining dark matter substructures with gravitationally lensed arcs.

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arXiv2013

Measuring cosmic distances with galaxy clusters

In addition to cosmological tests based on the mass function and clustering of galaxy clusters, which probe the growth of cosmic structure, nature offers two independent ways of using clusters to measure cosmic distances. The first uses measurements of the X-ray emitting gas mass fraction, which is an approximately standard quantity, independent of mass and redshift, for the most massive clusters. The second uses combined millimeter (mm) and X-ray measurements of cluster pressure profiles. We review these methods, their current status and the prospects for improvements over the next decade. For the first technique, which currently provides comparable dark energy constraints to type Ia supernova studies, improvements of a factor of 6 or more should be readily achievable, together with tight constraints on the mean matter density that are largely independent of the cosmological model assumed. Realizing this potential will require a coordinated, multiwavelength approach, utilizing new cluster surveys, X-ray, optical and mm facilities, and a continued emphasis on improved hydrodynamical simulations.

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arXiv2013

Mass profiles of Galaxy Clusters from X-ray analysis

We review the methods adopted to reconstruct the mass profiles in X-ray luminous galaxy clusters. We discuss the limitations and the biases affecting these measurements and how these mass profiles can be used as cosmological proxies.

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arXiv2010

X-ray microlensing in the quadruply lensed quasar Q2237+0305

We use archival data of NASA's Chandra X-ray telescope to compile an X-ray light curve of all four images of the quadruply lensed quasar Q2237+0305 (z=1.695) from January 2006 to January 2007. We fit simulated point spread functions to the four individual quasar images using Cash's C-statistic to account for the Poisson nature of the X-ray signal. The quasar images display strong flux variations up to a factor of ~4 within one month. We can disentangle the intrinsic quasar variability from flux variations due to gravitational microlensing by looking at the flux ratios of the individual quasar images. Doing this, we find evidence for microlensing in image A. In particular, the time-sequence of the flux ratio A/B in the X-ray regime correlates with the corresponding sequence in the optical monitoring by OGLE in the V-band. The amplitudes in the X-ray light curve are larger. For the most prominent peak, the increase of the X-ray ratio A/B is larger by a factor ~1.6 compared to the signal in the optical. In agreement with theory and other observations of multiply imaged quasars, this suggests that the X-ray emission region of this quasar is significantly smaller than the optical emission region.

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arXiv2008

Microlensing variability in the gravitationally lensed quasar QSO 2237+0305 = the Einstein Cross. II. Energy profile of the accretion disk

We present the continuation of our long-term spectroscopic monitoring of the gravitationally lensed quasar QSO 2237+0305. We investigate the chromatic variations observed in the UV/optical continuum of both quasar images A and B, and compare them with numerical simulations to infer the energy profile of the quasar accretion disk. Our procedure combines the microlensing ray-shooting technique with Bayesian analysis, and derives probability distributions for the source sizes as a function of wavelength. We find that the effective caustic crossing timescale is 4.0+/-1.0 months. Using a robust prior on the effective transverse velocity, we find that the source responsible for the UV/optical continuum has an energy profile well reproduced by a power-law R lambda^{zeta} with zeta=1.2+/-0.3, where R is the source size responsible for the emission at wavelength lambda. This is the first accurate, model-independent determination of the energy profile of a quasar accretion disk on such small scales.

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arXiv2008

The time-delay of the gravitationally lensed double quasar UM 673

The paper has been withdrawn because double checking and comparison with other data sets after the original submission showed that a broken R-band filter at the Maidanak telescope had affected our quasar monitoring observations in the years 2004 and 2005. They had led to partially spurious measurements, hence our original analysis and conclusions are not reliable.

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arXiv2008

Improved constraints on dark energy from Chandra X-ray observations of the largest relaxed galaxy clusters

We present constraints on the mean matter density, Omega_m, dark energy density, Omega_de, and the dark energy equation of state parameter, w, using Chandra measurements of the X-ray gas mass fraction (fgas) in 42 hot (kT>5keV), X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range 0.05<z<1.1. Using only the fgas data for the 6 lowest redshift clusters at z<0.15, for which dark energy has a negligible effect on the measurements, we measure Omega_m=0.28+-0.06 (68% confidence, using standard priors on the Hubble Constant, H_0, and mean baryon density, Omega_bh^2). Analyzing the data for all 42 clusters, employing only weak priors on H_0 and Omega_bh^2, we obtain a similar result on Omega_m and detect the effects of dark energy on the distances to the clusters at ~99.99% confidence, with Omega_de=0.86+-0.21 for a non-flat LCDM model. The detection of dark energy is comparable in significance to recent SNIa studies and represents strong, independent evidence for cosmic acceleration. Systematic scatter remains undetected in the fgas data, despite a weighted mean statistical scatter in the distance measurements of only ~5%. For a flat cosmology with constant w, we measure Omega_m=0.28+-0.06 and w=-1.14+-0.31. Combining the fgas data with independent constraints from CMB and SNIa studies removes the need for priors on Omega_bh^2 and H_0 and leads to tighter constraints: Omega_m=0.253+-0.021 and w=-0.98+-0.07 for the same constant-w model. More general analyses in which we relax the assumption of flatness and/or allow evolution in w remain consistent with the cosmological constant paradigm. Our analysis includes conservative allowances for systematic uncertainties. The small systematic scatter and tight constraints bode well for future dark energy studies using the fgas method. (Abridged)

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arXiv2008

The multiple quasar Q2237+0305 under a microlensing caustic

We use the high magnification event seen in the 1999 OGLE campaign light curve of image C of the quadruply imaged gravitational lens Q2237+0305 to study the structure of the quasar engine. We have obtained g'- and r'-band photometry at the Apache Point Observatory 3.5m telescope where we find that the event has a smaller amplitude in the r'-band than in the g'- and OGLE V-bands. By comparing the light curves with microlensing simulations we obtain constraints on the sizes of the quasar regions contributing to the g'- and r'-band flux. Assuming that most of the surface mass density in the central kiloparsec of the lensing galaxy is due to stars and by modeling the source with a Gaussian profile, we obtain for the Gaussian width 1.20 x 10^15 sqrt(M/0.1M_sun)cm < sigma_g' < 7.96 x 10^15 sqrt(M/0.1Msun) cm, where M is the mean microlensing mass, and a ratio sigma_r'/sigma_g'=1.25^{+0.45}_{-0.15}. With the limits on the velocity of the lensing galaxy from Gil-Merino et al. (2005) as our only prior, we obtain 0.60 x 10^15 sqrt(M/0.1Msun) cm < sigma_g' < 1.57 x 10^15 sqrt(M/0.1Msun) cm and a ratio sigma_r'/sigma_g'=1.45^{+0.90}_{-0.25} (all values at 68 percent confidence). Additionally, from our microlensing simulations we find that, during the chromatic microlensing event observed, the continuum emitting region of the quasar crossed a caustic at >72 percent confidence.

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arXiv2007

The dark matter halos of massive, relaxed galaxy clusters observed with Chandra

We use the Chandra X-ray Observatory to study the dark matter halos of 34 massive, dynamically relaxed galaxy clusters, spanning the redshift range 0.06<z<0.7. The observed dark matter and total mass (dark-plus-luminous matter) profiles can be approximated by the Navarro Frenk & White (hereafter NFW) model for cold dark matter (CDM) halos; for ~80 per cent of the clusters, the NFW model provides a statistically acceptable fit. In contrast, the singular isothermal sphere model can, in almost every case, be completely ruled out. We observe a well-defined mass-concentration relation for the clusters with an intrinsic scatter in good agreement with the predictions from simulations. The slope of the mass-concentration relation, c\propto M_vir^a/(1+z)^b with a=-0.45\pm0.12 at 95 per cent confidence, is steeper than the value a~-0.1 predicted by CDM simulations for lower mass halos. With the slope a included as a free fit parameter, the redshift evolution of the concentration parameter, b=0.71\pm0.52 at 95 per cent confidence, is consistent with the same simulations (b~1). Fixing a~-0.1 leads to an apparent evolution that is significantly slower, b=0.30\pm0.49, although the goodness of fit in this case is significantly worse. Using a generalized NFW model, we find the inner dark matter density slope, alpha, to be consistent with unity at 95 per cent confidence for the majority of clusters. Combining the results for all clusters for which the generalized NFW model provides a good description of the data, we measure alpha=0.88\pm0.29 at 95 per cent confidence, in agreement with CDM model predictions.

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arXiv2005

Microlensing in the double quasar SBS1520+530

We present the results of a monitoring campaign of the double quasar SBS1520+530 at Maidanak observatory from April 2003 to August 2004. We obtained light curves in V and R filters that show small-amplitude Δm~0.1 mag intrinsic variations of the quasar on time scales of about 100 days. The data set is consistent with the previously determined time delay of Δt=(130+-3) days by Burud et al. (2002). We find that the time delay corrected magnitude difference between the quasar images is now larger by (0.14+-0.03) mag than during the observations by Burud et al. (2002). This confirms the presence of gravitational microlensing variations in this system.

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arXiv2004

An improved approach to measuring H_0 using X-ray and SZ observations of galaxy clusters

We present an improved method for predicting the Sunyaev-Zeldovich (SZ) effect in galaxy clusters from spatially-resolved, spectroscopic X-ray data. Using the deprojected electron density and temperature profiles measured within a fraction of the virial radius, and assuming a Navarro, Frenk & White (1995) mass model, we show how the pressure profile of the X-ray gas can be extrapolated to large radii, allowing the Comptonization parameter profile for the cluster to be predicted precisely. We apply our method to Chandra observations of three X-ray luminous, dynamically relaxed clusters with published SZ data: RX J1347.5-1145, Abell 1835 and Abell 478. Combining the predicted and observed SZ signals, we determine improved estimates for the Hubble constant from each cluster and obtain a weighted mean of H_0=69\pm8km/s/Mpc for a cosmology with Omega_m=0.3 and Omega_Lambda=0.7. This result is in good agreement with independent findings from the Hubble Key Project and the combination of cosmic microwave background and galaxy cluster data.

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arXiv2004

Constraints on dark energy from Chandra observations of the largest relaxed galaxy clusters

We present constraints on the mean dark energy density, Omega_X and dark energy equation of state parameter, w_X, based on Chandra measurements of the X-ray gas mass fraction in 26 X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range 0.07 -1, the same data require w_X<-0.7 at 95 per cent confidence. Similar results on the mean matter density and dark energy equation of state parameter, Omega_m=0.24\pm0.04 and w_X=-1.20^{+0.24}_{-0.28}, are obtained by replacing the CMB data with standard priors on the Hubble constant and mean baryon density and assuming a flat geometry.

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arXiv2004

Mapping small-scale temperature and abundance structures in the core of the Perseus cluster

We report further results from a 191 ks Chandra observation of the core of the Perseus cluster, Abell 426. The emission-weighted temperature and abundance structure is mapped detail. There are temperature variations down to ~1 kpc in the brightest regions. Globally, the strongest X-ray surface brightness features appear to be caused by temperature changes. Density and temperature changes conspire to give approximate azimuthal balance in pressure showing that the gas is in hydrostatic equilibrium. Si, S, Ar, Ca, Fe and Ni abundance profiles rise inward from about 100 kpc, peaking at about 30-40 kpc. Most of these abundances drop inwards of the peak, but Ne shows a central peak, all of which may be explained by resonance scattering. There is no evidence for a widespread additional cooler temperature component in the cluster with a temperature greater than a factor of two from the local temperature. There is however evidence for a widespread hard component which may be nonthermal. The temperature and abundance of gas in the cluster is observed to be correlated in a manner similar to that found between clusters.

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arXiv2003

A preference for a non-zero neutrino mass from cosmological data

We present results from the analysis of cosmic microwave background (CMB), large scale structure (galaxy redshift survey) and X-ray galaxy cluster (baryon fraction and X-ray luminosity function) data, assuming a geometrically flat cosmological model and allowing for tensor components and a non-negligible neutrino mass. From a combined analysis of all data, assuming three degenerate neutrinos species, we measure a contribution of neutrinos to the energy density of the universe, Omega_nu h^2=0.0059^{+0.0033}_{-0.0027} (68 per cent confidence limits), with zero falling on the 99 per cent confidence limit. This corresponds to ~4 per cent of the total mass density of the Universe and implies a species-summed neutrino mass \sum_i m_i =0.56^{+0.30}_{-0.26} eV, or m_nu~0.2 eV per neutrino. We examine possible sources of systematic uncertainty in the results. Combining the CMB, large scale structure and cluster baryon fraction data, we measure an amplitude of mass fluctuations on 8h^{-1} Mpc scales of sigma_8=0.74^{+0.12}_{-0.07}, which is consistent with measurements based on the X-ray luminosity function and other studies of the number density and evolution of galaxy clusters. This value is lower than that obtained when fixing a negligible neutrino mass (sigma_8=0.86^{+0.08}_{-0.07}). The combination of CMB, large scale structure and cluster baryon fraction data also leads to remarkably tight constraints on the Hubble constant, H_0=68.4^{+2.0}_{-1.4} km/s/Mpc, mean matter density, Omega_m =0.31\pm0.02 and physical baryon density, Omega_b h^2=0.024\pm0.001, of the Universe.

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arXiv2003

A deep Chandra observation of the Perseus cluster: shocks and ripples

We present preliminary results from a deep observation lasting almost 200 ks, of the centre of the Perseus cluster of galaxies around NGC 1275. The X-ray surface brightness of the intracluster gas beyond the inner 20 kpc, which contains the inner radio bubbles, is very smooth apart from some low amplitude quasi-periodic ripples. A clear density jump at a radius of 24 kpc to the NE, about 10 kpc out from the bubble rim, appears to be due to a weak shock driven by the northern radio bubble. A similar front may exist round both inner bubbles but is masked elsewhere by rim emission from bright cooler gas. The continuous blowing of bubbles by the central radio source, leading to the propagation of weak shocks and viscously-dissipating sound waves seen as the observed fronts and ripples, gives a rate of working which balances the radiative cooling within the inner 50 kpc of the cluster core.

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arXiv2003

Cosmological constraints from the local X-ray luminosity function of the most X-ray luminous galaxy clusters

We present precise constraints on the normalization of the power spectrum of mass fluctuations in the nearby universe, sigma_8, as a function of the mean local matter density, Omega_m. Using the observed local X-ray luminosity function of galaxy clusters from the extended BCS and REFLEX studies, a mass-luminosity relation determined from Chandra and ROSAT X-ray data and weak gravitational lensing observations, and the mass function predicted by the Hubble Volume simulations of Evrard et al., we obtain sigma_8 = (0.508\pm0.019) Omega_m^-(0.253\pm0.024), with Omega_m < 0.34 at 68 per cent confidence. The degeneracy between sigma_8 and Omega_m can be broken using Chandra measurements of the X-ray gas mass fractions in dynamically relaxed clusters. Using this information and including Gaussian priors on the mean baryon density of the universe and the Hubble constant, we obtain sigma_8=0.695\pm0.042 and Omega_m=0.287\pm0.036, for an assumed flat LambdaCDM cosmology (marginalized 68 per cent confidence limits). Our results are in good agreement with some recent studies based on the local X-ray temperature function of clusters, the redshift evolution of the X-ray luminosity and temperature functions of clusters, early results from the Sloan Digitized Sky Survey, the most recent results from studies of cosmic shear, and combined analyses of the 2dF galaxy redshift survey and cosmic microwave background anisotropies.

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arXiv2002

Chandra temperature and metallicity maps of the Perseus cluster core

We present temperature and metallicity maps of the Perseus cluster core obtained with the Chandra X-ray Observatory. We find an overall temperature rise from ~3.0 keV in the core to ~5.5 keV at 120 kpc and a metallicity profile that rises slowly from ~0.5 solar to ~0.6 solar inside 60 kpc, but drops to \~0.4 solar at 120 kpc. Spatially resolved spectroscopy in small cells shows that the temperature distribution in the Perseus cluster is not symmetrical. There is a wealth of structure in the temperature map on scales of ~10 arcsec (5.2 kpc) showing swirliness and a temperature rise that coincides with a sudden surface brightness drop in the X-ray image. We obtain a metallicity map of the Perseus cluster core and find that the spectra extracted from the two central X-ray holes as well as the western X-ray hole are best-fit by gas with higher temperature and higher metallicity than is found in the surroundings of the holes. A spectral deprojection analysis suggests, however, that this is due to a projection effect; for the northern X-ray hole we find tight limits on the presence of an isothermal component in the X-ray hole, ruling out volume-filling X-ray gas with temperatures below 11 keV at 3 sigma.

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arXiv2002

Optical monitoring of the gravitationally lensed quasar Q2237+0305 from APO between June 1995 and January 1998

We present a data set of images of the gravitationally lensed quasar Q2237+0305, that was obtained at the Apache Point Observatory (APO) between June 1995 and January 1998. Although the images were taken under variable, often poor seeing conditions and with coarse pixel sampling, photometry is possible for the two brighter quasar images A and B with the help of exact quasar image positions from HST observations. We obtain a light curve with 73 data points for each of the images A and B. There is evidence for a long (>~ 100 day) brightness peak in image A in 1996 with an amplitude of about 0.4 to 0.5 mag (relative to 1995), which indicates that microlensing has been taking place in the lensing galaxy. Image B does not vary much over the course of the observation period. The long, smooth variation of the light curve is similar to the results from the OGLE monitoring of the system (Wozniak et al. 2000a).

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